The present invention is a system and method to control and optimize isomerization processes.
Such processes include the isomerization of a hydrocarbon feed to create a product (i.e., isomerate) whose economic value exceeds that of the feed material. Isomerization is generally carried out with a catalyst and under reaction conditions chosen to maximize conversion consistent with target product specifications. Plant operating strategies attempt to maximize the production of isomerate product within certain constraints such as feed rates, catalyst lifetimes, process parameters (e.g., temperature and pressure), and desired product properties which may be dependent upon the degree of conversion. Process models are sometimes developed and employed which permit the plant operator to optimize run lengths; thereby achieving maximum profitability within the practical constraints. Typically, the process is allowed to run for a predetermined time, either continuous or batch, established by experience and model calculations. This predetermined time (i.e., run-length) is not always optimum due to feed variability, imperfections in the process model and uncertainties in catalyst activity and actual process conditions. The net result is that the process may be terminated prematurely, or allowed to proceed longer than would be optimum. In the first instance, profitability is diminished since the catalyst and the capital-intensive plant equipment are not fully utilized. In the second instance, for example, by allowing the process to run for a time longer than is optimum, feed material will not be adequately converted into valuable isomerate product when the catalyst has exceeded its useful lifetime. The net result is the unwanted production of lesser-valued material which, when blended with the higher-valued material produced during the early stages of the run, reduce the quality (i.e., net value) of the final product. Additionally, storage costs are incurred for products having off-target specifications.
For the above stated reasons, there is significant economic advantage to be gained by operating isomerization processes under a control scheme which includes means to continuously monitor the process stream and estimate, from this input, the quality of the product or the effectiveness of the process, or both as a function of time. This control scheme comprises, in part, an analyzer that would initially monitor and analyze the first batch of product and a means for said analyzer to send a signal either to a computer or to a plant operator confirming that the process is yielding product with maximum available quality. For example, the analyzer would measure the degree to which feed has been converted to isomerized product (i.e., degree of conversion). As feed quality, process conditions, and catalyst activity vary due to either known or unknown causes, the analyzer will monitor the effectiveness of the conversion process and provide a signal to either a computer or a process operator that the run should be terminated. One object of the present invention is to provide process units with a means capable of determining the degree of conversion in an isomerization application, or other applications where plant economics can be optimized by continuously tracking the quality of the product.
Another aspect of the isomerization process is the separation of the isomerate product from the non-isomerized feed. Incomplete separation results in loss in yield of the higher valued isomerate, since the separated, non-isomerized feed is recycled and mixed with fresh feed. Consequently, means to determine the effectiveness of the separation and to control the separation process based upon this information has value and is another application of this invention to the isomerization system.
One specific, but not restrictive example of such a process involves the isomerization of a linear paraffin feed into a product consisting mainly of branched paraffins. More specifically, if the linear paraffins are of sufficiently high molecular weight, the feed will be a wax and the isomerate product will be an oil over a broad temperature range. For example, the feed may be a hydrocarbon which boils in the lubricating oil range. In this case, all of the above general arguments apply--the wax is catalytically isomerized into a higher-value isomerate oil. The analyzer described in the present invention provides information on the degree of conversion which, in this case, is directly determined by the relative amount of branched versus linear paraffins in the product. Complete conversion is rarely achieved in practice; hence, the product of the isomerization process acting upon a hydrocarbon wax feed, for example, will contain some unconverted wax. Thus, the product mixture is called a waxy isomerate. Since complete conversion is not achieved in practice, the analyzer will be used to maximize run-length and optimize process operations by reporting either to a computer or a process operator when the relative proportion of branched to linear paraffins has maximized. A plant optimization scheme could use this information, for example, to decide when to terminate the run; say at the time that the analyzer shows that the abundance of branched versus linear paraffins has either leveled-out, or shows the first signs of decline or to optimize operating conditions such as temperature and H.sub.2 pressure.
Since conversion of the wax to isomerate oil is not complete in practice it is necessary to separate the isomerate oil from the non-isomerized wax. This may be accomplished by catalytic or solvent dewaxing of the mixture. Two streams will emerge from the dewaxer, a dewaxed isomerate oil and a residual wax stream. The residual wax stream may contain some of the isomerate oil since the dewaxing process is also not complete, in practice. For example, in solvent dewaxing isomerate oil may be physically entrained in the wax. In this example, the efficiency of the separation process may be controlled by solvent type and amount relative to the wax-isomerate mixture feed, and flow rate to the separation filter. Means to provide rapid determination of the amount of the isomerate oil that may be entrained in the wax allows the operator to control and optimize the dewaxing process with respect to the reduction of isomerate oil yield and throughput.
The present invention claims a method for controlling and optimizing such processes as are typified but are not limited to isomerization processes in general and wax isomerization processes specifically. The present invention also claims a method which is both novel and unobvious whereby the quality of an isomerization process can be determined by spectroscopic means.